U.S. patent number 11,418,871 [Application Number 17/051,242] was granted by the patent office on 2022-08-16 for microphone array.
This patent grant is currently assigned to Sennheiser electronic GmbH & Co. KG. The grantee listed for this patent is Sennheiser electronic GmbH & Co. KG. Invention is credited to Alexander Kruger.
United States Patent |
11,418,871 |
Kruger |
August 16, 2022 |
Microphone array
Abstract
For certain application cases, such as e.g., in a sports
stadium, a microphone array having a particularly high directivity
in the vertical direction and a high, yet in wide limits adjustable
directivity in horizontal direction is provided. The microphone
array has a plurality of microphones whose output signals are
combined into at least one common output signal. The microphones
are directional microphones with a preferred direction of high
sensitivity and arranged substantially in one plane on a circle or
segment of a circle, such that each microphone has a different
direction of high directivity. For each of the microphones, the
preferred direction of high sensitivity is substantially orthogonal
to the circle or segment of the circle. A common output signal of
the microphone array is obtained by beamforming. The microphone
array has an adjustable preferred direction of high sensitivity,
wherein the common output signal comprises the sound recorded from
this adjustable direction.
Inventors: |
Kruger; Alexander (Hannover,
DE) |
Applicant: |
Name |
City |
State |
Country |
Type |
Sennheiser electronic GmbH & Co. KG |
Wedemark |
N/A |
DE |
|
|
Assignee: |
Sennheiser electronic GmbH &
Co. KG (Wedemark, DE)
|
Family
ID: |
1000006501909 |
Appl.
No.: |
17/051,242 |
Filed: |
May 6, 2019 |
PCT
Filed: |
May 06, 2019 |
PCT No.: |
PCT/EP2019/061529 |
371(c)(1),(2),(4) Date: |
October 28, 2020 |
PCT
Pub. No.: |
WO2019/211487 |
PCT
Pub. Date: |
November 07, 2019 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20210235187 A1 |
Jul 29, 2021 |
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Foreign Application Priority Data
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|
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May 4, 2018 [DE] |
|
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102018110759.5 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04R
1/326 (20130101); H04R 3/005 (20130101); H04R
2430/23 (20130101); H04R 2201/401 (20130101); H04R
2203/12 (20130101) |
Current International
Class: |
H04R
1/32 (20060101); H04R 3/00 (20060101) |
Field of
Search: |
;381/56,58,91,92,122,362,390 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1 538 867 |
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Jun 2005 |
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EP |
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3188504 |
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Jul 2017 |
|
EP |
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H0572025 |
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Mar 1993 |
|
JP |
|
WO 2009/009568 |
|
Jan 2009 |
|
WO |
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WO 2014/083542 |
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Jun 2014 |
|
WO |
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WO 2015/013058 |
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Jan 2015 |
|
WO |
|
Other References
Search Report for Application No. PCT/EP2019/061529 dated Jul. 12,
2019. cited by applicant .
Renato S. Pellegrini et al., "Object-Audio Capture System for
Sports Broadcast" Sep. 27, 2018 (Sep. 27, 2018). Retrieved from the
Internet: https://www.ibc.org/download?ac=6529 [retrieved on Jul.
2, 2019] XP055601525. cited by applicant .
Niwa et al., Binaural sound generation corresponding to
omnidirectional video view using angular region-wise source
enhancement, 2016 IEEE International Conference on Acoustics,
Speech and Signal Processing (ICASSP), Mar. 20, 2016. cited by
applicant .
Yo Sasak et al., Electroacoustics and Audio Engineering: Paper
ICA2016-155 Development of multichannel single-unit microphone
using shotgun microphone array, In: 22nd International Congress on
Acoustics--Sep. 5-9, 2016--Buenos Aires, 2016. cited by
applicant.
|
Primary Examiner: Jerez Lora; William A
Attorney, Agent or Firm: Haug Partners LLP
Claims
The invention claimed is:
1. A microphone array comprising a plurality of microphones whose
output signals are combined into at least one common output signal,
wherein the microphones are directional microphones, each
comprising a microphone capsule, an interference tube and a
preferred direction of high sensitivity, the interference tube
pointing in the preferred direction of high sensitivity; the
microphones are arranged substantially in one plane; the
microphones are arranged such that each microphone has a different
preferred direction of high sensitivity; the microphones are
arranged such that their microphone capsules are positioned on a
circle or segment of a circle and, for each of the microphones, the
preferred direction of high sensitivity is substantially orthogonal
to the circle or segment of the circle; the common output signal is
obtained by modal beamforming; and the microphone array has at
least one adjustable preferred direction of high sensitivity,
wherein the common output signal comprises the sound captured from
this at least one adjustable direction, wherein a directivity of
the microphone array in a dimension perpendicular to the plane of
the microphones substantially corresponds to the directivity of a
single directional microphone.
2. The microphone array according to claim 1, wherein the
beamforming generates a beam pattern of the microphone array, the
beam pattern being defined by a degree M, wherein a higher degree
means a more focused beam pattern, and wherein the
frequency-independent mixing matrix has (2M+1) outputs with
M.ltoreq.(Q-1)/2, Q being the number of microphones.
3. The microphone array according to claim 1, further comprising an
electronic circuit arrangement for processing the output signals of
the microphones to perform the modal beamforming.
4. The microphone array according to claim 3, wherein the
electronic circuit arrangement comprises at least the following
elements: a mixing matrix for mixing the microphone signals into
(2M+1) mixed signals, with M being the order of the common output
signal and M.ltoreq.(Q-1)/2, Q being the number of microphones; a
plurality of (2M+1) filters for filtering the mixed signals,
wherein filtered mixed signals are generated, and wherein the
filters perform a filtering that is adapted to the employed type of
directional microphone; a plurality of (2M+1) weighting units for
weighting each of the filtered mixed signals with a weighting,
wherein the weighting of each weighting unit corresponds to the
adjustable preferred direction of high sensitivity of the
microphone array; and a summation unit for summing up the (2M+1)
weighted, filtered mixed signals, wherein an output signal is
generated that comprises sound from the adjustable preferred
direction of high sensitivity of the microphone array.
5. The microphone array according to claim 4, wherein the weighting
units are first weighting units, and wherein the microphone array
has at least two preferred directions of high sensitivity and
wherein the electronic circuit arrangement comprises further
weighting units and at least one further summation unit, wherein
the second weighting units process the same filtered mixed signals
as the first weighting units, but receive different directional
information for the preferred direction of high sensitivity of the
microphone array.
6. The microphone array according to claim 1, wherein the
microphone capsules are arranged on a circle or segment of a circle
with a radius of between r.sub.min=5 cm and r.sub.max=100 cm.
7. The microphone array according to claim 6, wherein the radius is
between 30 cm and 60 cm.
8. The microphone array according to claim 1, further comprising a
control unit for adjusting the preferred direction of high
sensitivity of the microphone array or an input for connecting such
control unit.
9. The microphone array according to claim 1, wherein for each of
the microphones the preferred direction of high sensitivity points
outwardly relative to the circle or segment of a circle.
10. The microphone array according to claim 1, wherein for each of
the microphones the preferred direction of high sensitivity points
inwardly relative to the circle or segment of a circle.
11. The microphone array according to claim 1, wherein the
directional microphones are shotgun microphones.
12. A method for audio recording using a microphone array of
directional microphones, wherein at least one common output signal
is generated that comprises sound from an adjustable preferred
direction of high sensitivity of the microphone array and being
obtained through modal beamforming, and wherein each directional
microphone comprises a microphone capsule and an interference tube
pointing in a preferred direction of high sensitivity of the
respective directional microphone, the method comprising: mixing a
plurality of microphone signals received from the directional
microphones in a frequency-independent mixing matrix to obtain
(2M+1) mixed signals, wherein M is the order of the common output
signal, and wherein the directional microphones are arranged
substantially in one plane such that the microphone capsules are
arranged on a circle or segment of a circle and for each of the
directional microphones the preferred direction of high sensitivity
is substantially orthogonal to the circle or segment of a circle;
filtering the mixed signals in a plurality of (2M+1) filters,
wherein filtered mixed signals are generated; weighting each of the
filtered mixed signals with a weighting in a plurality of (2M+1)
weighting units, wherein the weighting of each weighting unit
corresponds to the adjustable preferred direction of high
sensitivity of the microphone array; and summing up the (2M+1)
weighted, filtered mixed signals in a summation unit, wherein the
common output signal is generated; wherein a directional
characteristic of the microphone array in a dimension perpendicular
to the plane of the microphones corresponds substantially to the
directional characteristic of a single directional microphone.
13. The method according to claim 12, further comprising: detecting
a change in an input signal, the input signal controlling the
adjustable preferred direction of high sensitivity of the
microphone array; and changing the preferred direction of high
sensitivity of the microphone array according to the detected
change.
14. The method according to claim 13, wherein the filtering
function of each filter depends on radial components of modal
responses of the employed type of microphones.
15. The microphone array according to claim 1, wherein a
frequency-independent mixing matrix is used for the modal
beamforming, and wherein output signals of the mixing matrix are
filtered, weighted and summed up, wherein the filtering function of
each filter depends on radial components of modal responses of the
employed type of microphones.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is the National Stage entry under 35 U.S.C. .sctn.
371 of International Application No. PCT/EP2019/061529 filed May 6,
2019, published as Publication No. WO 2019/211487 on Nov. 7, 2019,
which claims benefit of foreign priority of German Patent
Application No. 10 2018 110 759.5, filed on May 4, 2018, the
entireties of which are herein incorporated by reference.
FIELD OF DISCLOSURE
The invention relates to a microphone array.
BACKGROUND
For sound recordings in large sports facilities, the acoustic
events on the field may be particularly interesting for an
immersive playback, such as noise from the ball, the bat or racket
and so forth as well as conversations of the players, umpire or
referee, trainers and so forth. Due to the amount of ambient noise,
it is difficult to achieve a good sound quality and speech
intelligibility. This has to do with the fact that microphones
often have to be positioned on the edge of the field, because a
large distance to the desired sound sources needs to be maintained.
The disturbing noise comprises substantially noise of the audience,
which in sports facilities is normally found in the spectator
stands. Moreover, the microphones for sound recording should not
block the view for the spectators or the usually present
cameras.
A typical example is the playing field of a soccer stadium, wherein
ball noises, player conversations, whistling of the referee and
trainer instructions should be captured.
Similar problems may occur in other sports, such as, e.g.,
baseball, or in other situations where sound recordings are to be
made from sound sources that are widely distributed over a plane
area and that may be mobile and cannot be directly provided with a
microphone, despite disturbing ambient noise.
A solution from LAWO that is known as "KICK" is an arrangement of
numerous directional microphones or microphones having a
super-cardioid characteristic, which are distributed around a
soccer field on the edge of the field, parallel to the ground
(https://www.lawo.com/en/products/audio-production-tools/kick.html).
For capturing ball noises, the ball position is visually tracked,
automatically or semi-automatically. The position data are input
into an automatic audio mixing unit that receives also the
microphones' output signals, processes or weights them respectively
according to the position data and mixes them. The idea behind is
that signals from microphones that are closest to the current ball
position are particularly weighted. A disadvantage of this known
solution is that a large amount of cabling is required. The cables
and the microphones must be laid before each game and removed again
after the game. Additional microphones require additional cabling
and make the system more expensive. Further, due to the fixed
alignment of the microphones, their optimally captured region must
be relatively wide in order to cover also regions in between
neighboring microphones. Nevertheless, these regions are captured
with only poor sound quality and therefore suboptimal.
Additionally, a larger coverage area of the microphones in the
plane (azimuth angle) leads to an increase in the vertical coverage
area (elevation angle), since the directional characteristics
(i.e., beam patterns) of known microphones are rotationally
symmetric. This means that noises from the higher spectator stands
are also captured.
Another possible solution consists in a manual alignment or
tracking of directional microphones with a particularly high
directivity. However, this is associated with a time delay.
Moreover, service personnel for each directional microphone is
required in the case of manual alignment, and structure-borne noise
can be transferred to the microphone. With a possible remote
control for aligning the microphones, both additional delay and
motor noise would occur, which would inevitably be captured by the
microphone and be hearable as disturbing noise. An incorrect
alignment of a directional microphone affects different frequencies
differently, since the directivity of the directional microphones
is stronger for higher frequencies than for lower. This leads to a
permanently changing tone or timbre of the sound signal.
Another known solution for achieving a high directional effect is
beamforming, where output signals of a plurality of microphones
arranged as an array are combined, e.g., using delay, addition and
filtering. The resulting beam, i.e., the region of particularly
high sensitivity, has an adjustable direction and is usually
rotationally symmetric. The respective shape of the beam depends on
the type, number and arrangement of the microphones as well as on
the algorithm that is used for the combining. Common algorithms are
the Delay-and-Sum (DS) algorithm and the "Minimum Variance
Distortionless Response" (MVDR) algorithm, which both have
drawbacks, however. Normally, microphone arrays are constructed
from microphones without or with low directivity, since they are
easy to handle and cheap. This requires a very large number of
microphones for obtaining a high directivity over a wide azimuth
angle and a similar directivity with respect to elevation, leading
to a high computation effort.
It is therefore an object of the present invention to provide a
microphone arrangement that solves the above-mentioned
problems.
For multi-channel audio recording, e.g., for 22 channels, an
arrangement with shotgun microphones arranged on a circle is known
(Y. Sasaki, T. Nishiguchi, K. Ono: "Development of multichannel
single-unit microphone using shotgun microphone array").
Neighboring shotgun microphones are used for additionally narrowing
the rotationally symmetric directivity pattern (or beam pattern) of
each single shotgun microphone at low frequencies to the respective
direction by filtering. In another known solution (K. Niwa, Y.
Koizumi, K. Kobayashi, H. Uematsu: "Binaural sound generation
corresponding to omnidirectional video view using angular
region-wise source enhancement"), shotgun microphones are used as
an alternative to beamforming.
SUMMARY OF INVENTION
An object of the present invention is to provide a microphone
arrangement with a particularly high directivity in vertical
direction and a high yet in wide limits adjustable directivity in
horizontal direction.
This object is achieved by the microphone array according to claim
1.
According to the invention, a microphone array comprises a
plurality of microphones whose output signals are combined into at
least one common output signal, wherein the microphones are shotgun
microphones arranged with a preferred direction of high
sensitivity. Further, the microphones are arranged essentially
evenly on a circle or segment of a circle such that each of the
microphones has another preferred direction of high sensitivity,
wherein preferably the angles between the individual microphones
are substantially equal over the entire circle or segment. The
microphones may point inwardly or outwardly with respect to the
circle or circle segment. In one embodiment, all microphones are
arranged substantially in one plane. In another embodiment, the
microphones are arranged in multiple, e.g., two or three, parallel
and adjacent planes. The thickness of each plane may correspond to
about the diameter of a microphone or interference tube,
respectively. The common output signal of the microphone array is
obtained by beamforming.
Due to the high directivity of the shotgun microphones, both the
elevation angle and the azimuth angle of the detection area or
coverage of the arrangement are very small, while the azimuth angle
is adjustable in a very large range that may be up to 360.degree..
The resulting azimuthal directivity of the microphone arrangement
can be stronger than the directivity of a single shotgun
microphone, even if none of the shotgun microphones points to the
respective direction. In embodiments where the microphones are
distributed over a full circle, there are always some shotgun
microphones that point opposite to the actual target direction.
This enables a constant directivity, regardless of the orientation
of the microphone array.
A method for audio recording by means of shotgun microphones is
disclosed in 12. Further advantageous embodiments are disclosed in
the claims 2-11, 13-14 and in the following detailed
description.
BRIEF DESCRIPTION OF THE DRAWINGS
Further details and advantageous embodiments are depicted in the
drawings in which:
FIG. 1 shows a microphone array in a first embodiment;
FIG. 2 shows a shotgun microphone having an interference tube;
FIG. 3 shows a block diagram of a signal processing for the
beamforming algorithm;
FIG. 4 shows a microphone array in a second embodiment;
FIG. 5 shows a microphone array in a third embodiment;
FIG. 6 shows a microphone array in a fourth embodiment;
FIG. 7 shows a block diagram of a multi-focus signal processing for
the beamforming algorithm;
FIG. 8 shows a diagram of radial components of modal responses of a
Sennheiser MKH8070 shotgun microphone;
FIG. 9 shows a microphone array in a fifth embodiment; and
FIG. 10 shows a perspective view of a microphone array, in an
embodiment.
DETAILED DESCRIPTION
FIG. 1 shows exemplarily, in one embodiment of the invention, a
circular microphone array 100 with thirty-one directional
microphones 110, wherein the microphone array 100 as well as each
individual directional microphone 110 have a very high directivity.
Each of the directional microphones 110 comprises a microphone
capsule, wherein the microphone capsules of all directional
microphones 110 are arranged on a circle 120 with a radius r around
a center C. Further, each of the directional microphones 110
comprises an interference tube that is disposed orthogonally
relative to the circle 120 and is directed radially outwardly. The
interference tube ensures the directional characteristic of the
respective directional microphone. The microphones are therefore
also called shotgun microphones. The preferred direction of high
sensitivity of each shotgun microphone is in its respective
longitudinal direction, thus also orthogonally relative to circle
120 or, respectively, radially relative to the overall arrangement.
Thus, each microphone has a different preferred direction of high
sensitivity. The shotgun microphones are distributed over the
circle essentially uniformly, so that equal angles are between the
microphones respectively, e.g.,
360.degree./31.apprxeq.11.6.degree.. Further, all shotgun
microphones may be arranged in a common plane, in one embodiment.
The entire arrangement is positioned substantially horizontally,
e.g., in a soccer stadium, such that the shotgun microphones are
aligned parallel to the ground.
Alternatively, the shotgun microphones may be arranged in two or
more different planes. These planes should preferably be close to
each other. In principle, the microphones may also be arranged in
different planes, but the sensitivity of all microphones with
regard to a defined elevation should then be similar. In other
words, the "view angles" or focus regions of the various
microphones should all be substantially in one plane in an intended
distance.
The radius of the circle 120 or circle segment determines the alias
frequency and the operating frequency range. A larger radius, at a
constant number of directional microphones, results in improvements
for low frequencies, by leading to a shift of this range to lower
frequencies and leading to a lower alias frequency. Increasing the
number of microphones results in a higher alias frequency.
FIG. 2 shows exemplarily a single shotgun microphone 200 that may
be used as directional microphone 110 in the arrangement 100. The
shotgun microphone 200 comprises a tube 210 acting as an
interference tube with a microphone capsule 240 disposed therein
(not visible in the drawing). The microphone capsule may be
electrically connected via electrical connectors 250 at the rear
end of the shotgun microphone. The interference tube 210 comprises
in this example at its front end one or more openings 230 serving
for sound entrance. Disposed laterally and distributed over the
length of the tube there are further openings 220 that allow also
laterally arriving sound to pass into the tube. This laterally
arriving sound may enter the tube also through the openings 230,
but it is phase-shifted due to the longer path. In the tube, it is
superimposed with the lateral sound coming in through the side
openings 220. Due to interference within the tube, this sound is
therefore compensated, so that a lower sensitivity for laterally
arriving sound results. Only for frontally arriving sound the
components that enter the tube through openings 220, 230 are
constructively superimposed, which leads to a higher sensitivity of
the microphone for the frontally arriving sound ("endfire shotgun
microphone"). The side openings 220 of the interference tube are
normally not distributed over its circumference, but are located on
only one side which is referred to as upper side of the shotgun
microphone in the following.
Shotgun microphones afford the advantage of a particularly high
directivity, which relates both to a very small azimuth angle as
well as a very small elevational angle. The elevational angle is
the angle perpendicular to the drawing plane in FIG. 1. Although
the azimuth angle, i.e., the angle in the drawing plane of FIG. 1,
of each individual shotgun microphone is also very small, a
directivity of the entire arrangement in the plane can be
controlled by including adjacent shotgun microphones and performing
suitable calculations for combining the different microphone
signals. In particular, the directivity of a rotationally
symmetrical arrangement as in FIG. 1 can be electronically
controlled to any direction of the plane, i.e., to any azimuth
angle. The elevational angle of the directivity or beam pattern of
the entire arrangement is the same as the elevational angle of the
directivity or beam pattern of each individual shotgun microphone,
i.e., very small. Thus, it is not necessary to arrange microphones
in a plurality of vertical planes for obtaining a high vertical
directivity. This allows a flat arrangement, which e.g., in a
sports stadium does not disturb the spectators' or cameras' view if
the microphone array is positioned at the edge of the playing
field. Further, no calculations for a combination (which is
possibly time-varying) of the microphone signals over the vertical
axis are required.
A further advantage of a rotationally symmetrical arrangement as in
FIG. 1 is that the directivity as well as the frequency
characteristic is uniform in any direction of the plane, i.e., to
any azimuth angle. Therefore, no sound coloration of laterally
arriving sound occurs, such as e.g., noise from the audience, if
the direction of high sensitivity of the arrangement is changed.
Moreover, it is easy to define multiple directions simultaneously
as directions of high sensitivity by multiple parallel different
processing of the microphone signals. This allows the beam to be
focused to multiple azimuth angles simultaneously, i.e., multiple
sound sources may be recorded from different directions
simultaneously with high directivity.
Various methods of signal processing may be used. A possible and
particularly advantageous signal processing for the microphone
array is the beamforming algorithm. Here, the beamforming is based
on the so-called modal beamforming, which is especially suitable
for configurations where all microphones have essentially the same
directivity (directional effect) and are arranged on a sphere or on
a circle. For the operating frequency range of the array it is
possible to achieve an almost uniform directivity over all
frequencies of the operating frequency range. The number Q of
microphones used determines the maximum achievable degree M of the
output signal, which corresponds to the spatial resolution of the
beam pattern, according to
.ltoreq. ##EQU00001## The processing is effected in two steps: (a)
frequency-independent mixing (or matrixing) of the microphone
signals to obtain 2M+1 intermediate signals or mixed signals, and
(b) filtering and then weighting and summing of the intermediate
signals or mixed signals.
What is especially remarkable is the option to accomplish directing
the beam (i.e., steering the resulting direction of high
sensitivity) to a target azimuth angle .PHI..sub.T by accordingly
recomputing the real values weights g.sub.m.sup.(.PHI..sup.T.sup.).
The steering (i.e., providing information about the target azimuth
angle .PHI..sub.T) may be done either manually or automatically,
e.g., by a visual tracking system. It is of particular importance
that the actual steering of the microphone array is accomplished
electronically, i.e., contactlessly, and that the steering
information is time-variant. Further, the filtered signals are
weighted correspondingly before summation, which eases simultaneous
recording of multiple sound sources as targets. An example is shown
in FIG. 7 and explained hereinafter.
FIG. 3 shows a block circuit diagram of signal processing for the
modal beamforming algorithm for an array of directional microphones
arranged on a circle. The Q microphone signals X(.omega.,x.sub.1),
. . . , X(.omega.,x.sub.Q) are mixed in a transformation matrix
T.sup.(M)(.PHI..sub.1, .PHI..sub.2, . . . .PHI..sub.Q) 310 in a
frequency-independent manner. The transformation matrix is valid
for a desired maximum degree
.ltoreq. ##EQU00002## and provides (2M+1) output signals. Each
output signal is filtered, wherein one filter 320 of the (2M+1)
filters 320, . . . , 322' occurs once and all others occur twice as
equal filter pairs 321, 321'. E.g., the filter 321 for the
(-M+1).sup.th matrix output and the filter 321' for the
(M-1).sup.th matrix output are equal. Each filter or filter pair
respectively has its own filtering function, corresponding to an
order of a particular mode. The output signal of each filter 320, .
. . , 322' is weighted in one or more weighting units 330 according
to the desired azimuthal direction .PHI..sub.T with a corresponding
gain value g.sub.-M.sup.(.PHI..sup.T.sup.),
g.sub.-M+1.sup.(.PHI..sup.T.sup.), . . . ,
g.sub.M.sup.(.PHI..sup.T.sup.). The 2M+1 weighted filtered mixed
signals are summed up in a summation unit 340, and the sum signal
Y(.omega.) can then be either provided as output signal 360, or
optionally filtered in an equalization filter 350 and then output.
Thus, a very flexible time-variant beamforming is possible.
For the number of directional microphones and their positions, the
following applies. In general, the number of microphones determines
the spatial resolution of the achievable target beam pattern or
directional characteristic, in particular the maximum directivity
index, which is the ratio between the beamformer's output power
with respect to a desired target direction and the total output
power integrated over all other directions. In the context of modal
beamforming, it is useful to choose the number Q of microphones in
dependence of the required maximum degree M according to Q=2M+1. If
the Circular Harmonics transform described below is used, then it
is advantageous, in consideration of the assumptions made for it,
to use a uniform distribution of microphones on a circle. This
ensures a uniform signal quality over all (azimuthal) directions,
as intended with modal beamforming.
FIG. 4 shows a microphone array 400 in a second embodiment. Eleven
directional microphones 410.sub.1, . . . , 410.sub.11 are arranged
radially distributed uniformly over a circle 420. According to
Q=2M+1 with Q=11, a signal with a degree of at most M=5 can be
generated.
If another algorithm than modal beamforming is used, it may however
be appropriate to arrange the directional microphones differently,
namely not exactly radially but slightly rotated or displaced,
respectively. This makes the overall arrangement smaller, without
reducing the length of the individual directional microphones or
the diameter of the circle of microphone capsules. FIG. 5 shows a
microphone array 500 in a third embodiment, where each of the
eleven microphones 510.sub.1, . . . , 510.sub.11 is rotated through
an angle .alpha. with their microphone capsules being arranged on a
circle 520. The algorithm used must consider this rotation, wherein
very small angles can be neglected.
Moreover, it may make sense for certain applications to arrange the
directional microphones on a segment of a circle that has a certain
angle, e.g., if only low levels of disturbing noise from the rear
are to be expected. However, the disadvantage of a segmental
arrangement as compared to a circular arrangement is that for a
positioning near the edge, ambient noise from directions in which
no directional microphone is pointed cannot be well suppressed.
This problem can be compensated partially by making the segment
larger than the region to be observed. FIG. 6 shows a microphone
array 600 in a fourth embodiment, wherein again eleven directional
microphones 610.sub.1, . . . , 610.sub.11 are evenly distributed
over a semicircle. For a central alignment near 0.degree.
corresponding to the microphone 6106 this arrangement works well.
Also for a region of e.g. .+-.45.degree. around the central
alignment an acceptable result may be achievable. Correspondingly,
a microphone array of a form as shown in FIG. 6 is usable e.g., at
the corners of a playing field where a region or coverage range of
substantially 90.degree. is to be covered.
However, for segmental arrangements of directional microphones,
other algorithms than modal beamforming are normally better suited,
since they are not based on a circularly symmetrical arrangement of
the microphones. But a disadvantage of such alternatively usable
algorithms is that not only their scalar weightings but also their
filtering functions are direction dependent. Since the calculation
of filtering functions, or filter coefficients respectively, is
often relatively computationally expensive, these may be calculated
in advance. The device comprises then a memory in which the
respective filtering coefficients for certain directions are stored
and from which they can be retrieved if necessary. In this way,
real-time operation is also possible with such alternative
algorithms.
FIG. 7 shows a block diagram of a multi-focus signal processing for
the beamforming algorithm. Like the single-focus signal processing
already shown in FIG. 3, the multi-focus signal processing
comprises a mixing matrix 310 for mixing the microphone signals
into (2M+1) mixed signals, wherein M is the order of the common
output signal, and a plurality of (2M+1) filters 320, 321, 321',
322, 322' for filtering the mixed signals, wherein filtered mixed
signals QF.sub.-M, QF.sub.-M+1, . . . , QF.sub.0, . . . ,
QF.sub.M-1, QF.sub.M are generated. The filtered mixed signals are
now provided not only to (2M+1) first weighting units 330.sub.1,
but also to (2M+1) second weighting units 330.sub.2. The first
weighting units 330.sub.1 weight each of the filtered mixed signals
with a first weighting g.sub.-M.sup.(.PHI..sup.T1.sup.), . . . ,
g.sub.0.sup.(.PHI..sup.T1.sup.), . . . ,
g.sub.M.sup.(.PHI..sup.T1.sup.), and the second weighting units
330.sub.2 weight each of the filtered mixed signals with a second
weighting g.sub.-M.sup.(.PHI..sup.T2.sup.), . . . ,
g.sub.0.sup.(.PHI..sup.T2.sup.), . . . ,
g.sub.M.sup.(.PHI..sup.T2.sup.). Each of the first weightings
corresponds to the first preferred direction of high sensitivity 1'
T1 and each of the second weightings corresponds to the second
preferred direction of high sensitivity .PHI..sub.T2. The output
signals of the first weighting units 330.sub.1 and the output
signals of the second weighting units 330.sub.2 are added up
separately from each other in two separate summation units 3401,
3402, optionally filtered 3501, 3502 and then output. Accordingly,
the microphone array has two preferred directions of high
sensitivity, .PHI..sub.T1, .PHI..sub.T2 simultaneously. The two
output signals 3601, 3602 comprise the audio signals from these two
preferred directions of high sensitivity of the microphone array.
E.g., noise coming from a ball and from a referee may
simultaneously be extracted and recorded. An advantage of the
arrangement is that the second weighting units 330.sub.2 process
the same filtered mixed signals as the first weighting units
330.sub.1, using only different directional information for the
preferred direction of high sensitivity .PHI..sub.T2. Therefore,
the filters 320, . . . , 322' need to be calculated and implemented
only once, since they are direction independent. The weighting
units may be implemented e.g., by multipliers. The entire
arrangement shown in FIG. 3 or in FIG. 7 may be realized by one or
more microprocessors, which may be configured by corresponding
software programs.
Details of the two-dimensional modal beamforming will be explained
hereinafter.
First, basic assumptions and relationships will be explained. In a
compact area of interest in three-dimensional space that contains
the center of a notional coordinate system, which is free of sound
sources and which is excited from outside by a sound field which is
independent of the coordinate system's z-axis, there is an array of
Q acoustic sensors (i.e., microphones) that behave linearly. They
are arranged on a circle within the x-y plane of the notional
coordinate system, with the (two-dimensional) coordinates
.times..times..PHI..times..times..PHI..times..di-elect cons.
##EQU00003##
with r.sub.0 being the radius of the circle and .PHI..sub.q being
the azimuth angle of the q-th microphone, measured
counter-clockwise in the x-y plane from the x-axis. The frequency
domain representation X(.omega., x.sub.q) of the q-th microphone
signal at an angular frequency .omega. may be expressed as a
superimposition (or composition) of responses to individual plane
waves impinging from all possible azimuth angles .PHI., i.e.,
X(.omega.,x.sub.q)=.intg..sub.-.pi..sup..pi.H(.omega.,x.sub.q,.PHI.-
)C(.omega.,.PHI.)d.PHI. (2)
Here, C(.omega.,.PHI.) denotes the so-called plane wave amplitude
density function, which is basically a frequency domain
representation of the sound pressure in the coordinate origin
caused by a single plane wave incident from an azimuth angle .PHI..
H(.omega., x.sub.q, .PHI.) indicates the directivity pattern of the
q-th microphone.
By expanding the directivity pattern H(.omega., x.sub.q,.PHI.) and
the plane wave amplitude density function C(.omega., .PHI.) into
series of real valued orthonormal Circular Harmonics (a special
form of the Spherical Harmonics), defined by
.function..PHI..times..pi..times..function..times..times..PHI..times..tim-
es.>.times..times..times..function..times..times..PHI..times..times.<-
; ##EQU00004## according to
H(.omega.,x.sub.q,.PHI.)=.SIGMA..sub.m=-.infin..sup..infin.H.sub.m(.omega-
.,x.sub.q)trg.sub.m(.PHI.) (4)
C(.omega.,.PHI.)=.SIGMA..sub.m=-.infin..sup..infin.C.sub.m(.omega.)trg.su-
b.m(.PHI.) (5) and exploiting the orthonormality of the Circular
Harmonics, i.e.,
.intg..sub.-.pi..sup..pi.trg.sub.m(.PHI.)trg.sub.m,(.PHI.)d.PHI.=.delta..-
sub.m,m, (6) where .delta., denotes the Kronecker delta function,
the frequency domain microphone signal representation X(.omega.,
x.sub.q) can be reformulated as
.function..omega..intg..pi..pi..times..infin..infin..times..function..ome-
ga..times..times..times..function..PHI.'.infin..infin..times.'.function..o-
mega..times..times..times.'.function..PHI..times..times..times..PHI..times-
..infin..infin..times..function..omega..times..function..omega.
##EQU00005##
The individual weights H.sub.m(.omega., x.sub.q) of the Circular
Harmonics series in (4) are called the modal responses of degree
m.
If all microphones have identical directivity patterns and face
outwards or inwards perpendicular to the circle, this may be
formally expressed as
H(.omega.,x.sub.q,.PHI.)=H.sub.PROTO(.omega.,r.sub.0,.PHI.-.PHI..sub.q)
(9) with H.sub.PROTO(.omega., r.sub.0, .PHI.) indicating a
.PHI.-symmetric prototype directivity, which can be regarded as
belonging to a microphone located at a position (r.sub.0,
.PHI..sub.q=0). Due to its .PHI.-symmetry, the Circular Harmonics
expansion of H.sub.PROTO(.omega., r.sub.0, .PHI.) is given by
H.sub.PROTO(.omega.,r.sub.0,.PHI.)=.SIGMA..sub.m=-.infin..sup..infin.H.su-
b.PROTO,m(.omega.,r.sub.0)trg.sub.m(.PHI.) (10) with
H.sub.PROTO,m(.omega.,r.sub.0)=0 for m<0. (11)
For this special case, the modal responses can be factorized into a
frequency and radius-dependent component and another component that
only depends on the azimuth angle according to
.times..function..omega..function..omega..times..times..times..function..-
PHI..times..times..times..function..omega..pi..times..times..times..times.-
.function..omega..times..times.>.times..pi..times..times..times..times.-
.function..omega..times..times..pi..times..times..times..times..function..-
omega..times..times.< ##EQU00006##
Further remarkable is the symmetry of the radial components
b.sub.m(.omega.,r.sub.0)=b.sub.-m(.omega.,r.sub.0).A-inverted.m
(14) and the fact that the radial components depend on the product
of the angular frequency and the radius:
b.sub.m(.omega.,r.sub.0)=b.sub.m(.omega.r.sub.0) (14a)
By plugging (12) into (8), the frequency domain representation
X(.omega., x.sub.q) of the q-th microphone signal may be express as
X(.omega.,x.sub.q)=.SIGMA..sub.m=-.infin..sup..infin.b.sub.m(.omega.,r.su-
b.0)C.sub.m(.omega.)trg.sub.m(.PHI..sub.q) (15)
In the following, the basic principle of modal beamforming is
described. It may be subdivided into the following two steps: (1)
reconstructing from the microphone signals X(.omega., x.sub.q) the
underlying composition of the incident sound field of individual
plane waves represented by the Circular Harmonics series expansion
coefficients C.sub.m(.omega.) of the plane wave amplitude density
function, and (2) weighting the individual plane waves of the
incident sound field according to a desired target beam pattern,
and subsequently their integration in order to obtain the output
signal of the beamformer.
A block diagram of a typical modal beamformer is shown in FIG. 3
and FIG. 7, as described above. The two mentioned steps will be
described in more detail in the following.
To motivate the incident sound field reconstruction, the Circular
Harmonics expansion of the frequency domain microphone signals
X(.omega.,x.sub.q)=.SIGMA..sub.m=-.infin..sup..infin.X.sub.m(.omega.,r.su-
b.0)trg.sub.m(.PHI..sub.q) (16) is compared with (15). It becomes
clear that the expansion coefficients X.sub.m(.omega., r.sub.0) are
related to the desired Circular Harmonics series expansion
coefficients C.sub.m(.omega.) of the plane wave amplitude density
function according to
X.sub.m(.omega.,r.sub.0)=b.sub.m(.omega.,r.sub.0)C.sub.m(.omega.)
(17)
Therefore, two further steps are performed: (1) The Circular
Harmonics series expansion coefficients of the frequency domain
microphone signals are estimated by a Circular Harmonics transform
according to {circumflex over
(X)}.sub.m(.omega.,r.sub.0)=.SIGMA..sub.q=1.sup.Qw.sub.qX(.omega.,x.-
sub.q)trg.sub.m(.PHI..sub.q) (18)
Here, it is to be noted that due to the finite number Q of spatial
sampling points x.sub.q the maximum absolute value of the degree m
that can be reconstructed is also finite, and depends on the
distribution of the spatial sampling points x.sub.q on the circle.
For instance, for the special case of a uniform distribution, the
weights are all equal, namely
.times..pi. ##EQU00007## and the maximum absolute value of the
degree m that can be reconstructed is given by
.times..times. ##EQU00008##
By defining the vector X(.omega.) containing the signals of all
microphones by X(.omega.)=[X(.omega.,x.sub.1)X(.omega.,x.sub.2) . .
. X(.omega.,x.sub.Q)].sup.T (20)
the vector with all Circular Harmonics series expansion
coefficients by X.sub.CH(.omega.,r.sub.0)=[{circumflex over
(X)}.sub.-M(.omega.,r.sub.0){circumflex over
(X)}.sub.-M+1(.omega.,r.sub.0) . . . {circumflex over
(X)}.sub.M(.omega.,r.sub.0)].sup.T (21)
and the discrete Circular Harmonics transformation matrix by
.function..PHI..PHI..times..PHI.
.function..PHI..function..PHI..function..PHI..function..PHI..function..PH-
I..function..PHI. .function..PHI..function..PHI..function..PHI.
##EQU00009##
the estimation of the Circular Harmonics series expansion
coefficients may be expressed by the following matrix
multiplication:
X.sub.CH(.omega.,r.sub.0)=T.sup.(M)(.PHI..sub.1,.PHI..sub.2, . . .
,.PHI..sub.Q)X(.omega.) (23)
Especially important is that this matrix is frequency independent.
(2) Considering (17) and (14), the Circular Harmonics series
expansion coefficients of the plane wave amplitude density function
are estimated in principle as follows:
.function..omega..function..omega..function..omega..times..function..omeg-
a..function..omega. ##EQU00010##
which corresponds to a filtering operation for each individual
estimated Circular Harmonics series expansion coefficient of the
frequency domain microphone signals {circumflex over
(X)}.sub.m(.omega., r.sub.0)
Using the estimated Circular Harmonics series expansion
coefficients of the plane wave amplitude density function, the
individual plane waves of the incident sound field are weighted
according to a desired target beam pattern to be subsequently
integrated, or summed up respectively.
The maximum degree M of the Circular Harmonics series expansion
coefficients of the plane wave amplitude density function
determines the maximum possible spatial resolution of the target
beam pattern. Hence, a prototype of a desired target beam pattern
is defined by means of a Circular Harmonics expansion truncated at
the same maximum degree M:
g.sup.(.PHI..sup.T.sup.=0)(.PHI.)=.SIGMA..sub.m=0.sup.Mg.sub.m.sup.(.PHI.-
.sup.T.sup.=0)trg.sub.m(.PHI.) (26) which is steered towards a
target azimuth angle .PHI..sub.T=0 and which is .PHI.-symmetric.
Due to the symmetry, the expansion coefficients for negative degree
indices m are zero.
If the target beam pattern is steered to an arbitrary target
azimuth .PHI..sub.T, its Circular Harmonics series expansion
coefficients can be computed in dependence on those for
.PHI..sub.T=0 according to
g.sub.m.sup.(.PHI..sup.T.sup.)=cos(m.PHI..sub.T)g.sub.m.sup.(.PHI..sup.T.-
sup.=0)+sin(m.PHI..sub.T)g.sub.-m.sup.(.PHI..sup.T.sup.=0).A-inverted.m.di-
-elect cons.{-M, . . . , M} (27)
The actual frequency domain output signal Y(w) of the beamformer is
computed as weighted sum of Circular Harmonics series expansion
coefficients of the plane wave amplitude density function as
follows:
Y(.omega.)=.SIGMA..sub.m=-M.sup.Mg.sub.m.sup.(.PHI..sup.T.sup.)C.sub.m(.o-
mega.) (28)
Due to the equivalence of (28) with
Y(.omega.)=.intg..sub.-.pi..sup..pi.g.sup.(.PHI..sup.T.sup.)(.PHI.)C(.ome-
ga.,.PHI.)d.PHI. (29) the integration of the weighted plane wave
contributions to the incident sound field becomes evident.
For most applications, the frequency-invariant beam pattern used
above is advantageous and desired. However, also a frequency
dependent beam pattern can be created very easily by making the
weighting factors frequency dependent. This requires a filter per
individual Circular Harmonics series expansion coefficient of the
plane wave amplitude density function before summation.
Optionally, an equalizing filter 350, 350' can be applied to the
output signal Y(.omega.) of the beamformer to create a direction
independent coloration, or compensate a direction dependent
coloration respectively, e.g., to attenuate high frequency signal
components affected by spatial aliasing.
The radius of the circle on which the microphone capsules of the
directional microphones are arranged affects at least two
parameters of the array, namely the practically realizable
directivity for low frequencies and the frequency at which the
spatial aliasing starts occurring.
The directivity at low frequencies is affected as follows. The
radial components b.sub.m(.omega., r.sub.0) of the modal responses
typically have a high-pass characteristic, where the cutoff
frequency increases with the degree index m. For illustration, FIG.
8 shows exemplarily a diagram of magnitudes of radial components of
modal responses for various degrees m of a Sennheiser MKH8070
shotgun microphone, plotted over a product .omega.r.sub.0. As can
be seen, the contributions of modes with increasing degree m within
the measured microphone signals (16) become very small, in
particular for low spectral frequencies. Hence, reconstructing the
corresponding Circular Harmonics series expansion coefficients of
the plane wave amplitude density function requires a high
amplification factor of
.function..omega. ##EQU00011## (see (26), since |b.sub.|m|(.omega.,
r.sub.0)| is small. This leads to a typically low white noise gain
when using a target beam pattern of high degree m, which means that
microphone noise is highly amplified within the beamformer output
signal. By increasing the radius r of the array, the curves
depicted in FIG. 8 substantially are shifted to the left, i.e.,
towards lower frequencies. This leads to a decrease of the
high-pass cutoff frequencies and thereby reduces the effect of
white noise amplification at low frequencies, compared to a smaller
radius.
Spatial aliasing is a phenomenon that occurs e.g., when sampling a
sound field with the sampling points being distributed too sparsely
to capture high frequency spatial sound pressure oscillations.
Since the relevance of Circular Harmonics with higher degree m
within the signature function usually grows with spectral
frequency, the same happens with the amount of error caused by the
spatial aliasing. In particular, the angular frequency where the
contribution of Circular Harmonics of degrees greater than M to the
signature function becomes significant can be seen as the frequency
where the aliasing error effects start to become disturbing, or
notable respectively. Substantially, this angular frequency is
.omega. ##EQU00012## where cs denotes the speed of sound. This
means that for a chosen number Q of microphones the spatial
aliasing frequency may be increased by decreasing the array radius
r. Alternatively, the number of microphones can be increased for a
given array radius.
For microphone arrays for audible frequencies, the microphone
capsules should be on a circle or circle segment with a radius of
at least r.sub.min=5 cm. For practical reasons, a maximum radius of
about r.sub.max=100 cm is advisable. For microphone arrays intended
for usage in a sports stadium it is advantageous if for outwardly
pointing shotgun microphones the radius is between r.sub.min=30 cm
and r.sub.max=40 cm, and for inwardly pointing shotgun microphones
e.g., between r.sub.min=40 cm and r.sub.max=60 cm. With the
exemplarily described arrangement, a very high directivity e.g.,
for frequencies in the range of 200 Hz-3 kHz can be achieved. For
recordings in a sports stadium, frequencies below 3-4 kHz are
particularly relevant.
A smaller construction of the microphone array is possible if the
circular arranged shotgun microphones point radially inward. The
above calculations continue to be valid in this case. FIG. 9
schematically shows, in a fifth embodiment, a microphone array 900
with eleven shotgun microphones with the individual shotgun
microphones 910.sub.1, . . . , 910.sub.11 being aligned
substantially towards the center C of the array. The respective
microphone capsules (not shown) are positioned on a circle 920 with
the radius r. Using a radius of r=50 cm and e.g., Sennheiser
MKH8070 shotgun microphones with a length of about 46.5 cm (wherein
the microphone capsule is about 6 cm from the rear end), the
diameter of the entire array is therefore only 2*(50+6) cm=112 cm
instead of 2*(50+40.5) cm=181 cm.
FIG. 10 shows, in a further embodiment, a perspective view of a
similar microphone array 1000 with fifteen shotgun microphones
1010.sub.1, . . . , 1010.sub.15 that are also aligned towards the
center C of the array. The microphones may be attached e.g., to a
ring or a plate. It is particularly important to ensure that the
lateral openings 220 of the interference tubes 1010.sub.1, . . . ,
1010.sub.15 must not be covered, since they are the most important
sound entrance here. Thus, the shotgun microphones 1010.sub.1, . .
. 1010.sub.15 are not disturbed by the respective opposite shotgun
microphone (i.e., in "view direction"). The shotgun microphones
1010.sub.1, . . . , 1010.sub.15 are therefore arranged such that
their upper sides with the lateral openings 220 are freely
accessible to the sound and preferably all point into the same
direction. As in the previously described examples, the shotgun
microphones 1010.sub.1, . . . , 1010.sub.15 are located
substantially in one plane, wherein the directivity of the
microphone array can be electronically steered within this plane.
It is to be noted that the illustration in FIG. 10 is not
necessarily true to scale. E.g., the microphones 1010.sub.1, . . .
, 1010.sub.15 should be distributed over the circle 1020 as evenly
as possible.
A particular advantage of the microphone array according to the
invention is that it needs not be moved but remains stationary,
wherein the direction of maximum sensitivity can be adjusted by
electronic control, in the case of the circular arrangement to any
direction withing the plane of the circle (corresponding to an
azimuth angle of 0.degree.-360.degree. in a horizontal setup). In
other specific applications it may make sense to position the
circle vertically in order to capture an elevation angle of
0.degree.-360.degree. while keeping the azimuth angle very small.
Likewise, arbitrary orientations of the microphone plane are
possible in between. As shown in the drawings, there is no
microphone in the center of the arrangement. The mentioned
respective number of shotgun microphones per array is the
respective minimum number; it is always possible and may be
advantageous to increase the number Q of microphones, as explained
above. The number Q may be even or odd.
In an embodiment, the invention relates to a method for audio
recording by means of a microphone array composed of directional
microphones, wherein at least one common output signal is generated
that comprises sound coming from an adjustable preferred direction
of high sensitivity of the microphone array, with the steps: mixing
a plurality of microphone signals in a mixing matrix to obtain
(2M+1) mixed signals, wherein M is the order of the common output
signal, and wherein the microphone signals come from the
directional microphones and the directional microphones are
arranged substantially in a plane and on a circle or segment of a
circle, such that for each of the directional microphones a
preferred direction of high sensitivity is substantially orthogonal
outward or inward to the circle or circle segment; filtering the
mixed signals in a plurality of (2M+1) filters, wherein filtered
mixed signals are obtained, weighting each of the filtered mixed
signals with a weighting in a plurality of (2M+1) weighting units,
wherein the weighting of each weighting unit corresponds to the
adjustable preferred direction of high sensitivity of the
microphone array, and summing up the (2M+1) weighted filtered mixed
signals in a summation unit, wherein the common output signal is
obtained.
The embodiments described above are exemplary and may be combined
with one another, even if such combination is not expressly
mentioned. E.g., in an array arrangement as shown in FIG. 5, the
individual directional microphones may point inwardly, as in FIG. 9
and FIG. 10.
* * * * *
References